Projection, Heads, and Optimality

This article argues that inversion of the subject and auxiliary in English matrix questions and elsewhere is the effect of a violable optimality-theoretic constraint that requires head positions to be filled. When no other auxiliary is available, do-support occurs to satisfy this constraint, resulting in the presence of an expletive verb. When a higher-ranked constraint prohibits inversion, no inversion or do-support is found. The argument is then extended to cases where the complementizer that is obligatory, which are shown to offer best satisfaction of the proposed set of violable constraints.